1. Field of the Invention
This invention relates to a mechanism for feeding solid blocks of hot melt material toward a melting chamber of a hot melt applicator.
2. Description of the Related Art
Hand-held hot melt applicators as well as stationary hot melt applicators have a melting chamber that is adapted to receive and melt a forward end portion of an elongated, solid block of hot applied adhesive, sealant or similar material. The melting chamber has an outlet which dispenses molten material to a work site as additional portions of the block are fed into the melting chamber.
During use of many types of conventional hand-held hot melt adhesive applicators, thumb pressure is applied against the trailing end of the block of adhesive to advance the block toward the melting chamber while the fingers of the same hand grasp a handle of the applicator. In recent years, however, applicators having a feeding mechanism for guiding the blocks of solid adhesive toward the melting chamber have become available. As one example, the applicator described in U.S. Pat. No. 4,621,748 has a feeding mechanism with a drive member that, when depressed by an operator's thumb, moves a drive rack toward a position interleaved with a series of teeth formed in the block of adhesive in order to engage and shift the block forward toward the melting chamber.
A common problem of conventional hot melt adhesive applicators is the tendency for molten adhesive to drip from the nozzle for some time after the operator has relieved the forward pressure on the block. In such situations, the operator must move the applicator to prevent excess adhesive from reaching the work site, and position the nozzle of the applicator over a drip pan to prevent damage to the work area. Even more serious, however, is the potential for the molten adhesive to burn the user while dripping from the nozzle after the intended dispensing operation.
One solution proposed to overcome the problem of post-dispensing adhesive dripping is the provision of a check valve placed within the nozzle of the applicator. However, it is difficult to select a satisfactory, reliable spring for such a check valve because an overly stiff spring increases back pressure and hinders precise hand control of the dispensing operation while the block is fed into the melting chamber, while an overly weak spring may not close the valve in all instances.
Certain hot melt applicators have a feeding mechanism that is operable to retract remaining solid portions of the adhesive block after a dispensing operation. As one example, some stationary hot melt applicators have a feeding mechanism with a pair of fixed axis drive rollers which continuously engage and indent one side of the block. The drive rollers are connected to a reversible motor that initially moves the block toward the melting chamber to dispense molten adhesive, and then away from the melting chamber to suck back molten adhesive from the nozzle tip and provide room in the chamber for subsequent thermal expansion of a portion of the adhesive which remains in the chamber.
As another example, U.S. Pat. No. 4,379,516 describes a hand-held applicator with a clamping member that releasably grips a block of adhesive as the block is advanced, and the member is molded with a pair of resilient wings which retract the member along with the block at the end of a dispensing operation. Rearward movement of the clamping member shown in U.S. Pat. No. 4,379,516 ceases when the wings push the member into a position of contact with a rear wall of the applicator.
However, the distance that the afore-mentioned devices retract the block at the end of a dispensing operation must be carefully selected. If the retraction distance is too small, insufficient space for subsequent thermal expansion may result and the pressure of the expanding adhesive may force molten adhesive from the nozzle. On the other hand, if the retraction distance is too large, the forward end of the block may cool excessively and additional time or energy will be necessary before molten adhesive can replace the voids in the heat block and the dispensing operation can resume Moreover, if the retraction distance is too large, oxidation of the adhesive may be accelerated and an undue amount of air bubbles may be present in the extruded adhesive.